Thursday, September 25, 2014

Lights, cilia, action!

I didn't mention that the heteropods have a larval stage that looks very different than the adult. And although I've talked about veligers (the larval stage of most marine snails) before, e.g., see post from 11 September 2014, this veliger is noticeably different.

Check out those lobes! Many veligers have a velum (a lobed, highly ciliated structure surrounding the head that is used for swimming and feeding), but it's unusual for the velum to have 6 lobes. When we saw this veliger, we knew it was something different right away, just because of the spectacular multi-lobed velum.

Note that the shell of the veliger is different than the shell of the adult. It's much more rounded (see below):

But just like the adult, the larva can withdraw completely (velum and all) inside the shell:

This veliger was discovered by Emily after a plankton tow off Bodega Head on 9 September 2014. Eric has kept it alive in the lab since then and we've seen it developing and growing.

Thiriot-Quiévreux (1973) provided a helpful table with three stages of development in an atlantid heteropod. As development proceeds, the shell, velum, eyes, and foot all become more complex (see below).

When we first looked at it, the veliger was in Stage II — two of the lobes were much smaller than the others, the eye spots were relatively small and rounded, and the foot wasn't that noticeable.

However, within 9 days, we saw dramatic changes as the veliger transitioned to Stage III. The smaller lobes grew substantially so that all 6 lobes on the velum are now of equal size. The eye spots have become more visible and are now teardrop-shaped. And the foot has grown as it develops into a swimming fin.

Below are two images so you can see the changes in the velum (the growth of the smaller lobes). And then you'll get to watch a video with the veliger in action!

Velum with two smaller lobes (they look like the thumbs on a pair of mittens):

Velum with all 6 lobes of equal size:

And here's the highlight video — it shows the atlantid heteropod veliger actively swimming. Watch for all of the things discussed above — the long cilia beating along the edges of the velum, the rounded shell, and the transition from having 2 smaller lobes to all 6 lobes of equal length. By the way, this veliger is less than 0.5 mm long, so we're thankful that a microscope video camera can provide such nice views of this tiny whirling wonder!